1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile and in particular to an image forming apparatus having a fixing device for fixing an image onto a recording material.
2. Description of Related Art
In an electrophotographic image forming apparatus, a recording sheet having a toner image passes through a nip portion consisting of a heating unit and a pressure roller unit while the toner image is fixed onto the recording sheet. When the recording sheet remains in the nip portion, i.e., when a sheet jam is caused in the nip portion and jam clearance is tried, the jamming sheet cannot be pulled out because of the pressure of the nip portion.
For this, conventionally, a pressure release mechanism has been used to release the pressure of the fixing nip portion for facilitating jam clearance when jam has occurred.
However, in a fixing device having the pressure release mechanism, when a printer main body is operated with the fixing nip portion in a pressure released state, the toner image is not sufficiently fixed to the recording sheet, which causes contamination of the heating unit, a pressure roller unit, a conveying roller, and the like by an unfixed toner image. Moreover, in case of a heating unit using a ceramic heater, when the main body is operated with the pressure of the nip portion in a pressure released state, the heater may be damaged.
To solve such a problem, Japanese Patent Application Laid-Open No. 11-296017 discloses a technique as follows.
The technique uses a pressure release detection mechanism interlocked with the release operation of the fixing roller and a pressure roller. When a pressure release lever of the pressure release mechanism which is disposed on one side of the fixing device is moved to a pressure release direction, a cam member fixed to a rotation shaft which rotates together with the pressure release lever presses the pressure roller downward against a spring force of a compression spring, thereby releasing the pressure of the fixing roller and the pressure roller. Moreover, the rotation shaft extends almost over the entire length of the pressure roller. As the rotation shaft rotates, a protrusion provided on the rotation shaft rotates a sheet discharge sensor. This rotation of the sheet discharge sensor is transferred to a photo-interrupter arranged outside of the pressure roller to block the light path, thereby detecting the pressure release state.
The aforementioned conventional example is advantageous for pressure release detection but can be improved in the following points.
That is, in the conventional example, as has been described above, the pressure release lever arranged on only one side is operated. When the pressure release lever is operated and release operation is performed at both ends, the rotation shaft should be provided over the entire length of the roller and accordingly, the rotation shaft should have sufficient rigidity enabling release of the pressure and need be made from an expensive material.
Moreover, since the photo-interrupter is arranged outside the pressure roller and the protrusion for detecting a recording sheet of the discharge sensor unit is at a distance from the photo-interrupter, there is a problem that the discharge sensor unit becomes larger and the entire apparatus becomes larger in size.
It is therefore an object of the present invention to provide an image forming apparatus that can maintain a non-operation state of the apparatus main body without using a high-rigid expensive shaft member upon a pressure release.
Another object of the present invention is to provide an image forming apparatus capable of reducing the apparatus size and can maintain the non-operation state of the apparatus main body upon a pressure release.
Yet another object of the present invention is to provide an image forming apparatus comprising: fixing means having a nip portion; pressure applying means for applying a pressure to the nip portion; pressure release means for releasing the pressure in the nip portion; and detection means for detecting a presence or absence of a recording material, wherein a recording material having an unfixed image is nipped and conveyed by the nip portion and the unfixed image is fixed to the recording material, when the pressure in the nip portion is released by the pressure release means, the detection means outputs a detection result indicating that a recording material is present, and the pressure release means have a plurality of levers for releasing the pressure and the levers are arranged at both ends of the fixing means in a direction perpendicular to a conveying direction of the recording material.
Still another object of the present invention is to provide an image forming apparatus comprising: fixing means having a nip portion; pressure applying means for applying a pressure to the nip portion; pressure release means for releasing the pressure in the nip portion; and detection means for detecting a presence or absence of a recording material, wherein a recording material having an unfixed image is nipped and conveyed by the nip portion and the unfixed image is fixed to the recording material, when the pressure in the nip portion is released by the pressure release means, the detection means outputs a detection result indicating that a recording material is present, and the detection means is arranged within a width of the fixing means in a direction perpendicular to a conveying direction of the recording material.
These and other objects of the present invention will become clear from the following description.